How to overcome NAT challenges for client notifications

Notifications are usually sent by pushing messages from the application server to the client applications. Most event-driven architectural models such as PubSub, WebHooks, etc., especially those that work on the principles of HTTP/REST expect the subscriber’s endpoint (destination URL) to receive incoming HTTP POST requests from external sources.

Network Address Translation (NAT) is a technique ISPs use to translate private IP addresses to public IP addresses (especially when working with IPv4 addressing scheme) to enable multiple devices to share a single public IP address.

Typically, the implementation of the EDA is independent of the specific network infrastructure or ISP configuration. However, when we are sending notifications to the end users by using models like Pubsub or WebHooks, NAT can introduce challenges for establishing reliable communication if an ISP firewall blocks or filters these incoming requests.

To resolve this issue, we work with the ISP to allow incoming HTTP POST requests to the designated endpoints or URLs used for pushing events. If that’s not possible, the following are some approaches we can consider to overcome NAT challenges for sending notifications to clients.

Tunneling techniques

We can implement tunneling techniques such as UDP hole punching or TCP port forwarding. These techniques involve establishing a direct communication channel between a client and server behind a NAT by exploiting certain features of NAT traversal. However, their implementation can be more complex and can require additional infrastructure or protocols.

Reverse proxy

We can also set up a reverse proxy server in a public network that forwards EDA API requests to the clients. The reverse proxy acts as an intermediary, accepting incoming connections and relaying them to the clients. In this way, the reverse proxy can forward EDA traffic to the appropriate clients, allowing communication to traverse NAT boundaries.

Outbound communication

If server push events are not a strict requirement, we can use long polling as an alternative. In long polling, the client sends a request to the server, and the server holds the connection open until new data is available or a timeout occurs. This approach can work through NAT as it relies on the client-initiated request. In long polling, the client sends a request to the server, which keeps the connection open until it has new data to send back. This approach helps overcome NAT limitations as it keeps the connection open for a longer duration, reducing the chances of NAT routing issues.

Changing communication protocol

Consider utilizing WebSockets instead of HTTP/REST for real-time communication. WebSockets provide full-duplex communication channels between clients and servers, enabling bidirectional communication. WebSockets can bypass NAT limitations by establishing a persistent connection that allows real-time data transmission without being affected by NAT configurations.

Hybrid approach

We can combine different techniques to overcome NAT challenges. For example, we can use a combination of reverse proxy, long polling, and WebSocket to handle different scenarios and optimize connectivity. If the above options are unsuitable, we can also consider third-party services acting as intermediaries, relaying messages between our server and external clients, and effectively bypassing NAT restrictions.

Conclusion

Keep in mind that the choice of approach depends on the unique requirements, infrastructure, and constraints we face. By carefully evaluating available options and considering factors such as security, scalability, and maintenance, we can determine the best solution to address the NAT challenges in EDA APIs.